The invention relates to communications systems and, more particularly, to communications systems including radio transmission links.
Recent developments in the regulatory environment and in computer and communications circuit technology have brought great advances in personal and business communications. Significant changes have occurred in existing services, as a multitude of companies enter the long distance telephone market in competition with American Telephone and Telegraph, Inc. and as improved marine radiotelephone service and public telephone service in airplanes and trains have become available. Moreover, entirely new services are also now available to communications users, including cellular telephone service and radio determination satellite service (RDSS).
Such new and improved services provide valuable benefits to communications users. However, many such services have significant shortcomings in protecting the privacy of conversations carried over the service. For example, it is well known that marine radiotelephone circuits can be easily monitored by all other marine radiotelephone users, as well as by owners of scanning receivers readily available on the consumer market. It is not so widely known that cellular telephone communications are also readily receivable on scanning receivers as well as on many older television receivers having the capability to tune through channel 82 of the UHF band. It is also less widely known that many long distance telephone calls placed over standard long distance carriers travel at some point over radio links such as satellite circuits and terrestrial microwave relay circuits. A microwave receiver placed in the line of sight of such circuits, either between the stations of a circuit or directly behind one of the stations, can easily monitor the content of communications over such circuits.
Existing techniques are known for providing privacy over communications circuits which incorporate radio links. For example, scrambler/descrambler units are commercially available and can be added to cellular telephones and standard telephones communicating with cellular telephones. Such systems do not require interconnection to telephone company equipment, but limit privacy calls to those fixed telephone stations which are equipped with scrambler/descrambler units.
Other systems using sophisticated encryption technology allow calls to be placed from a cellular telephone to any fixed telephone, without requiring each called telephone to be equipped with a descrambler. Such a system is described, for example, in U.S. Pat. No. 4,549,308 to LoPinto. However, such systems require interconnection and modification to telephone company equipment and thus provide little flexibility for the user.
It is desirable to provide a method and apparatus for ensuring privacy of communications over circuits having radio links. It is also desirable to provide a method and apparatus to allow privacy for calls placed from a mobile telephone to any called telephone, without requiring additional equipment to be located at each called telephone. It is further desirable to provide the above-mentioned capabilities with minimum modification of equipment owned by the telephone company or other common carriers. It is also desirable to provide the capability to insure a selectable level of privacy in communication consistent with the confidentiality of information conveyed and financial resources available.